1. Field of the Invention
This invention relates generally to a control system for an AC motor and more particularly to an AC motor control system having an inverter that supplies AC power to the AC motor.
2. Description of the Prior Art
FIG. 1 shows one example of the conventional control system of an AC motor. In FIG. 1, reference numeral 10 designates a three-phase AC source, 11 a breaker, 12 a converter which includes thyristors, 13 a smoothing direct current (DC) reactor, 14 a 120.degree.-conduction inverter which includes thyristors, 15 an AC motor which is the load of the inverter 14, 151 a field winding of the AC motor, 152 a field converter that supplies power to the field winding 151, 16 a speed setter, 24 a speed detector that detects the speed of the AC motor, 17 a speed controller, 18 a current controller, 19 a phase controller that controls an output voltage of the converter 12, 20 a current detector, 23 an overlapped angle detector that detects an overlapped angle .mu. of the inverter 14, 22 a voltage detector, and 21 a control lead angle .beta. controller, respectively.
As can be seen from FIG. 1, AC power from the three-phase AC power source 10 is converted into a variable DC voltage by means of the converter 12, is smoothed by the DC reactor 13, thereafter is reversely converted into an AC power by the inverter 14, and then is supplied to the AC motor 15. The converter 12 and the inverter 14 are controlled such that a speed reference signal set by the speed setter 16 and a speed signal corresponding to the speed of the AC motor 15, as detected by the speed detector 24, are fed into the speed controller 17, and compared and amplified therein so as to produce thereby a current reference signal. The thus produced current reference signal and a current signal detected by the current detector 20 are compared and amplified within the current controller 18. Then the result of this comparison is fed into the phase controller 19 so as to control an output voltage of the converter 12.
On the other hand, the overlapped angle detector 23 detects an overlapped angle .mu. of the inverter 14 on the basis of currents, voltages and frequencies detected by the current detector 20. The thus detected overlapped angle .mu. and output voltage of the inverter 14 detected by the voltage detector 22 are fed into the control lead angle .beta. controller 21, which in turn, determines firing pulse timing for respective thyristors of the inverter 14 so as to control the output frequency thereof. Here, a control lead angle .beta. is based on a .gamma. constant control which is given by the sum of a constant commutation marginal angle .gamma. determined by the thyristors employed in the inverter 14 and an overlapped ang1e .mu. that increases in relation to an increase in current of the AC motor 15. More particularly, .gamma.=.omega.T.sub.c, where .omega. equals the angular frequency and T.sub.c equals the time from when the thyristor forward current becomes zero to the time when the inverse voltage becomes zero. The relationship is shown in FIG. 2.
When the above-described control is performed digitally using a microprocessor and the like, an overlapped angle .mu., which represents the time expressed in electrical degrees during which current is on simultaneously in two rectifying elements (.beta.=.gamma.+.mu.), is calculated with a specified calculation processing interval. However, if motor acceleration or deceleration is rapid or a change rate of current is great, the actual overlapped angle .mu. is varied more than the calculated value, whereby a commutation marginal angle becomes insufficient. This results in commutation failure of the inverter, so that a safety operation cannot be secured. This disadvantage can be eliminated by decreasing the time interval for processing of the overlapped angle .mu.. However, a microprocessor or the like inevitably has limited processing speed. Further, there have been such disadvantages that even if an overlapped angle is accurately processed, in the case when current of the AC motor 15 is rapidly increased, the terminal voltage is also varied causing an actual overlapped angle .mu. to be greater than the thus calculated overlapped angle .mu., so that the variable speed control of the AC motor 15 becomes unstable.